Optimizing the sampling strategy for an early classical swine fever detection

1. Department of Reproduction, Obstetrics, and Herd Health Faculty of Veterinary Medicine, Ghent University • Data from experiment in slaughter pigs (Dewulf et al., 2001): pen 1 (n=14), pen 2 (n=15), inoculation with CSF (pen 2, one animal, 0 days post inoculation (dpi)). • Virus Isolation (VI) and leukocyte count. • Random sampling and selection (apathy, leucopenia). • Probability of finding at least one positive pig in a pen (99% certainty) calculated using the hypergeometric distribution P (x > 0). ? N: total population x: number of successes (‘infected’) in sample n: sample size S: number of successes in population Optimizing the sampling strategy for an early classical swine fever detection S. Ribbens1, J. Dewulf1, F. Koenen2, H. Laevens2 and A. de Kruif1 1 Department of Obstetrics, Reproduction and Herd Health, Veterinary Epidemiology Unit, Faculty of Veterinary Medicine, Ghent University, Salisburylaan 133, 9820 Merelbeke, Belgium 2 Department of Virology, Unit for modelisation of epidemiological diseases, and coordination center for veterinary diagnostics, Veterinary and Agrochemical Research Centre (CODA/CERVA/VAR), Groeselenberg 99, 1180 Brussels, Belgium INTRODUCTION The early detection of a classical swine fever (CSF) epidemic is crucial to limit the duration of the high risk period (HRP). The length of this period is directly related to the extent of an outbreak. For an early detection of the CSF virus, antigen detecting tests are crucial. Yet these techniques are labour intensive and time consuming. Therefore it is crucial to limit the number of samples. This can be achieved by appropriate selection of animals that are to be sampled, or by selection of blood samples that are to be tested in the lab. MATERIALS AND METHODS RESULTS Random sampling and a priori selection based on apathy • Detection in first infected pen is possible from 3 dpi onwards (VI). • Until 12 dpi, no reduction in sample size is possible (sample size=entire pen). • From 17 dpi, substantial reduction in sample size when selected animals showed apathy. • Detection in adjacent pen only possible starting 23 dpi (data not shown). Selection of samples to be analyzed using the leucocyte count Necessary sample size (x-axis) in pen 2 • First samples with leucopenia were present from 15 dpi. • Similar reduction in sample size. • Sample size reduces with increasing number of infected animals in time. At least oneinfected (99% certainty) 21 dpi 21 dpi 100% 90% 80% 70% 7 dpi 60% Probability 50% 40% 7 dpi 30% 20% 0 dpi 10% 0% 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 0 dpi Sample size CONCLUSION DISCUSSION When no clinical symptoms are present, almost all animals in a pen are to be sampled. Once diseased animals are present, a priori selection of pigs based on apathy is appropriate. Therefore, monitoring for symptoms is crucial. • A sampling scheme to MAXIMIZE THE PROBABILITY OF DETECTING AT LEAST ONE CSF infected animal as soon as possible was developed: • early stage of infection (prevalence=low), no obvious clinical symptoms available • high proportion of animals to be sampled (± whole herd= unfeasible) . • appearance of clinical symptoms (up till 17 days), selection of animals based on symptoms is possible (apathy, fever (data not shown)). • within group of clinical diseased animals, a limited sample is sufficient to detect CSF. • Selection of blood samples to be analyzed is possible based on the leukocyte count with a reduction of samples starting 15 dpi. • In case of suspicion of a herd, monitoring for the appearance of clinical symptoms is crucial. This research was supported by the Belgian Ministry of Public Health, Research Foundation and the Fund for Health and Quality of Animals and Derived Products